1. Field of the Invention
The present invention relates to a multi-layered electrostrictive effect element and a method for producing the element.
2. Description of the Prior Art
As mechatronics techniques have been developed, there has been an increased demand for the supply, in large quantities and at a low price, of small-sized and light elements capable of precise fine positioning or those capable of providing a high output force and having a high response speed. Most of the conventional elements are those which make use of magnetic force and are represented by, for instance, motors, solenoids or voice coils, but recently a ceramics element which utilizes the electrostrictive properties have been developed and has attracted special interest. Among these, the elements which make use of an electrostrictive vertical effect have particularly attracted special interest since they can provide a high output force and have a higher response speed. However, the distortion of electrostrictive materials due to this effect is only in the order of 10.sup.-3 even under an electric field of 10 KV/cm. For this reason, there has been proposed an idea which comprises, for the purpose of increasing the displacement, alternatively superimposed electrodes and thin layers of an electrostrictive material which are mechanically in series and electrically in a parallel relation and it has been put into practical use in a specific field.
Among these multi-layered electrostrictive vertical effect elements, those proposed in a relatively early stage of this field are called stacked elements. For instance, such a multi-layered element which comprises 100 sheets of an electrostrictive material having a thickness ranging from 0.5 to 1 mm and electrodes having the same shape alternatively superimposed in layers provide a displacement of the order of only about 100 .mu.m even when a high voltage of the order of about 1000 V is applied thereto (see, for instance, Shuji YAMASHITA, Jpn. J. Appl. Phys., 1981, Suppl. 20, pp. 93-95). This stacked element is relatively large in size and a high voltage must be applied to the element in order to operate it. Therefore, these elements are greatly limited in applications.
On the other hand, Japanese Patent Unexamined Publication (hereinafter referred to as "J.P. KOKAI" No. Sho 58-196068 (TAKAHASHI et al.) proposes a method for producing an element having the same structure observed in the stacked elements, which comprises making use of the green sheet technique to reduce the thickness of each sheet of an electrostrictive material and forming a number of electrodes (inner electrodes), each sandwiched between the layers of the electrostrictive materials on the entire surface of the electrostrictive material. The element can be operated at a low voltage. To industrially produce such elements, it is necessary to electrically communicate every two layers to one another at the ends of the inner electrodes which are exposed at the side end face of the element. J.P. KOKAI No. Sho 59-115579 (OCHI et al.) proposes a solution of the foregoing problem which comprises electro-depositing a glassy inorganic material only on the exposed portions of the inner electrodes and the electrostrictive material adjacent to the exposed portions according to an electrophoresis method to form an insulating layer and to thus form an independent insulating layer for each inner electrode. It is said that, according to this method, the thickness of the layer of the electrostrictive material can be reduced to 0.1 to 0.2 mm and that the driving voltage can be reduced to a level of the order of 100 V. However, in such a method, there still remain a variety of difficulties for accomplishing a highly reliable insulation and thus the same applicant has proposed a variety of improvements concerning the electrophoresis method. Moreover, many attempts have been directed to methods for insulating the inner electrodes other than the electrophoresis method.
These proposals aim at the miniaturization and low voltage-driving of such an element, but there has not yet been proposed any method for reducing the thickness of the electrostrictive material to not more than 0.1 mm.
The multi-layered electrostrictive vertical effect element would be able to be operated at a voltage lower than that for the conventional element if the thickness of the electrostrictive material is further reduced. To this end, it is necessary to finely and precisely form insulating layers for insulating the end of inner electrodes exposed on the side end face of the element. However, if the exposed ends of the inner electrodes are insulated in every two layers to thus form an independent insulating layer for each layer of inner electrode, the inner electrodes which should not be insulated for the reasons such as the restriction concerning the operation and wettability of the insulating material and the electrostrictive material are possibly insulated as the thickness of the electrostrictive material decreases. As a result, inner electrodes which are not electrically connected to an external electrode would be formed and correspondingly, a desired quantity of displacement cannot be achieved.
The inventors of this invention conducted various studies on insulating materials as a means for solving the foregoing problems and have already developed a method comprising the steps of depositing a polyamide acid at the exposed end portions of the inner electrodes through electrophoresis to form a coating layer and then applying heat to convert the polyamide acid resin in the coating layer into a polyimide resin to form an insulating layer of the polyimide resin and have already filed a patent application (see Japanese Patent Application Serial No. Hei 1-171854). The method of this patent application is an epoch-making technique in that it relates to an insulation with an organic material, the use of which has conventionally been considered to be impossible and that it makes it possible to reduce the thickness of each layer of a multi-layered product to 100 .mu. or less. However, bubbles are liable to be captured in the insulating layer formed according to this method and dielectric breakdown is caused at the portions wherein such bubbles are captured. This leads to a great scattering in the withstand voltage of the resulting products.